I've read that anyone can request a post-grant patent review (assuming you have the money to cover the administrative costs) for as long as a patent is enforceable but I can't find any information on how long the review process takes.
Assuming you are talking about Post-Grant Patent Review as was newly introduced in the AIA, 35 U.S.C. §§ 321-329, there isn't a specific time frame for the review process; this page explains it fairly simply: http://www.postgrantproceedings.com/patent_modification/post_grant_review.html
So, it is similar to a litigation in a sense, in that once you file the request for a PGR, there's a 3 month period in which the patent owner can provide a preliminary response. Once the USPTO authorizes a PGR (after the three month period, IF that is, they decide there's enough evidence to say that "more likely than not" there is one or more claims that are unpatentable), then the patent owner and the petitioner go back and forth with evidence, motions to amend the patent, stays, mergers, estoppel, settlements, terminations, etc. just like a traditional litigation in most respects.
The main difference here is that the Patent Owner is not allowed to introduce new material to the patent, or expand its scope during the proceedings, so the likelyhood of the "litigation" being drawn out forever is lower than it would be with traditional litigation.
It is also worth noting that it is much more difficult to be granted a PGR than say, an IPR, which has a lower standard for granting an IPR proceeding "due to some of the issues that can be raised in PGR, such as enablement and § 101 invention issues, that may require development through discovery."